Crude steel is produced primarily by two methods; i.e., a blast-furnace/converter process and an electric furnace process. Of these methods, according to the electric furnace process, steel is manufactured by using scrap and reduced iron as iron feed stock, heating and melting them with electrical energy, and then refining the melted iron in some cases. At the present, scraps is a main feed stock. Recently, however, demand for reduced iron has increased due to a reduction in supply of scrap and a trend toward manufacture of high-grade products using the electric furnace process.
As one of known processes for producing reduced iron, Japanese Unexamined Patent Publication No. 63-108188 discloses a process comprising the steps of stacking a layer formed of iron ore and a solid reducing material on a hearth rotating horizontally, and heating the iron ore with radiant heat transfer from above to thereby reduce it. In many cases, the hearth rotating horizontally is constructed as shown in FIGS. 1 (a) and (b) which are explanatory view of a rotary hearth furnace. A layer 16 formed of iron ore and a solid reducing material is stacked on a moving (rotary) hearth 6 by using a charging apparatus 12. The moving hearth 6 is surrounded by a furnace body 13 including refractories set on its inner surface. A burner 14 is installed in an upper portion of the furnace and serves as a heat source for reducing the iron ore on the moving hearth 6. A temperature in the furnace is usually held at 1300.degree. C. or thereabout. Also, it is usual that, after the end of the reducing operation, the reduced iron is cooled with a cooler while remaining on the moving hearth, and then recovered for the purposes of preventing oxidization of the reduced iron outside the furnace and ensuring easier handling in subsequent steps. The above-stated process is advantageous because, for example, relatively less troubles occurred during the operation. Meanwhile, from the standpoint of aiming to improve productivity with the temperature in the furnace kept constant, the productivity of reduced iron in the above-stated process greatly depends on the amount of ore stacked on the hearth per unit area, the residence time of the ore in the furnace, and the area of the hearth. If the amount of ore stacked on the hearth per unit area can be increased, i.e., if the thickness of the ore layer can be enlarged, an output is expected to increase. In the case of heating the ore layer from only its upper surface as with the above-stated process, however, a very long time is required to heat the ore layer until its lower portion is heated; hence it is impossible to increase the productivity actually. Also, shortening of the residence time of the ore in the furnace contributes to increasing the output, but if the residence time is simply shortened, the ore is not sufficiently reduced and cannot convert into reduced iron. For those reasons, there has been a continuing problem that, to increase an output of reduced iron per one unit of equipment, a considerably large hearth area is required and large-scale equipment is needed.